This invention relates generally to wide-band direct-coupled transistor amplifiers of the type described in U.S. Patent Ser. No. 4,358,739 to Nelson, and more specifically to improved implementations of such amplifiers to achieve reduced settling time characteristics. One embodiment of the wide-band direct-coupled transistor amplifier described in the Nelson patent is illustrated in FIG. 1 of the present application. In that embodiment, an input voltage V.sub.1 produces an in phase amplified voltage V.sub.3, and another input voltage V.sub.2 produces a voltage V.sub.3 that is 180 degrees out of phase therewith, such that V.sub.3 =A(V.sub.1 -xV.sub.2), where A and x are factors that are functions of the circuit component values, the temperature of the components, the source and load impedances, the supply voltages .+-.V.sub.cc, and the magnitude and frequency of the input voltages. All of these factors have an adverse influence upon the settling time of the amplifier, which may be precisely defined for a linear system such as this, as being the time required, following the initiation of a step voltage for V.sub.1 or V.sub.2 or both, for the output to enter and remain within a specified narrow band centered on its steady state value.
Many techniques known in the art for reducing settling time in other types of amplifiers are obvious and are applicable to the amplifier of FIG. 1. For example, precision matched resistors having low temperature coefficients of resistance are useful. In addition, the use of monolithic transistors for Q0x and Q1x creates good thermal coupling so that the long term temperature of both transistors will be very close. This prevents the occurrence of large thermally induced V.sub.be mismatches. Likewise, in accordance with the instant FIG. 2, a diode connected transistor Q3x, thermally coupled to Q2x, may be coupled in series with RC1x to prevent a long term change in the collector current of Q2x as a function of temperature. A straightforward approach to reducing the effect of load impedance on amplifier settling time is to add a unity gain buffer B1 in series with the output, a difference which is also illustrated in FIG. 2. Lastly, to reduce the effect of V.sub.cc on settling time, transistors having a greater Early Voltage rating can be used and the value of RF2 can be increased. While these known techniques for reducing settling time are somewhat effective, their incorporation into amplifiers of the type taught by the Nelson reference does not result in a sufficiently reduced settling time characteristic for many applications.
It is, therefore, the principal object of the present invention to provide a wide-band direct-coupled transistor amplifier having a greatly improved settling time characteristic. This object is accomplished in accordance with the illustrated embodiments of this invention by modifying an amplifier of the type taught by Nelson to allow the use of current feedback rather than voltage feedback in order to reduce the sensitivity of settling time and bandwidth to feedback elements without thereby affecting the manner in which feedback is applied externally by the user, to reduce the sensitivity of settling time to the effects of temperature, to eliminate saturation and turn-off problems within the amplifier that are related to bias control, to large input signals, and to high frequency input signals or those having fast rise times, and to minimize the sensitivity of settling time to supply voltages.